1. Field of the Invention
This invention relates to a control device which controls a laser irradiation device to record visible information, such as characters, on a medium by irradiation of a laser beam. More particularly, this invention relates to a laser irradiation device control device which is adapted to minimize damages of a recording surface of a rewritable or write-once medium after visible information, such as characters, is recorded on the medium by irradiation of a laser beam.
2. Description of the Related Art
In recent years, a laser irradiation device (or a laser marker) is marketed and this laser irradiation device employs the technology which records characters, numbers, symbols, etc. on a medium by irradiating the medium with a laser beam. By irradiation of a laser beam, the medium absorbs the laser beam and is thereby heated, so that visible information, including characters, numbers, and symbols, is recorded on the medium.
The laser irradiation device includes a laser light source, such as a gas laser, a solid state laser, a liquid laser, a semiconductor laser, etc. By selecting an oscillation wavelength of a laser beam of the laser light source, the laser irradiation device can record the visible information on the medium, such as a metal, a plastic, a thermal paper, a thermal rewritable medium, etc.
When a metal or plastic is used as the medium, the metal or plastic is heated by irradiation of the laser beam to burn or eliminate the metal or plastic so that the visible information is printed on the metal or plastic. On the other hand, when a thermal paper or thermal rewritable medium is used as the medium, the thermal paper or thermal rewritable medium is heated by irradiation of the laser beam to color a recording layer of the thermal paper or thermal rewritable medium so that the visible information is printed on the thermal paper or thermal rewritable medium.
A thermal paper may be selected as a medium on which a delivery address or a name of an article of goods is printed. For example, such a thermal-paper type medium is stuck on a container made of a plastic used at a factory. The thermal-paper type medium is colored when the medium is heated. By using a thermal head or the like, characters or symbols can be recorded on the thermal-paper type medium.
In recent years, a thermal rewritable medium has been proposed which enables repeated writing and erasing of information. When using the medium in the field of logistics, it is desirable that writing and erasing of information is allowed for the medium stuck on the container. For this reason, the method of drawing characters on the medium by irradiating the medium with a laser beam and heating the medium in a non-contact manner has been proposed. For example, refers to Japanese Laid-Open Patent Publication No. 2004-090026. Japanese Laid-Open Patent Publication No. 2004-090026 discloses a relay lens system which is composed of a plurality of lenses connected by a plurality of flexible joints, and which transmits an image by a laser beam entered from one end to the other end.
Conventionally, the image formation by laser irradiation is known. For example, refer to Japanese Laid-Open Patent Publication No. 2004-341373. Japanese Laid-Open Patent Publication No. 2004-341373 discloses a method of forming an image in which image data of an original image is divided into a plurality of lines and a photoconductor drum is irradiated with a laser beam for each line so as to form an image on the photoconductor drum.
The thermal rewritable medium has the characteristic in which coloring of the medium is erased with the heating at a certain temperature and coloring of the medium appears again if the heating of the medium is applied further. However, if excessive heating is applied, the thermal rewritable medium will be deteriorated, which will shorten the life of the medium or will degrade the medium so as to disable erasing of information completely.
A description will be given of drawing of a character on a thermal rewritable medium. The thermal rewritable medium is heated by tracing strokes of a character by a laser beam, and an image of the character appears on the medium. FIG. 1 illustrates an example of a character drawn on a thermal rewritable medium by laser irradiation. The character illustrated in FIG. 1 is a modified numeral character “7”, and the strokes cross each other at an intersection 210. Because the laser irradiation to draw the following stroke is applied to the intersection 210 while the heat of the previously drawn stroke still remains, the intersection 210 is heated excessively and an undesired influence appears on the thermal rewritable medium.
Moreover, in the numeral character “7” illustrated in FIG. 1, a turn-back part 220 exists. Because of the influence of the inertia of the mirror for controlling the irradiation direction of a laser beam, the laser irradiation is applied near the turn-back part 220 for a comparatively long time, and the thermal rewritable medium is heated excessively at the turn-back part 220, and the thermal rewritable medium may be damaged at this point.
To avoid the problem, the method for preventing overlapping of laser irradiation has been proposed. For example, refer to Japanese Laid-Open Patent Publication No. 2006-306063, Japanese Patent No. 3990891, and Japanese Laid-Open Patent Publication No. 2008-179135.
Japanese Laid-Open Patent Publication No. 2006-306063 discloses a recording method in which scanning of a laser beam is performed so that at an intersection where two scanning lines cross each other, after the previous scanning line is passed, the following scanning line is drawn.
Japanese Patent No. 3990891 discloses a recording/erasing device which controls the laser irradiation part so that, when two laser drawing lines cross each other at an intersection, at least one of the laser irradiation power and the laser irradiation time to irradiate the intersection is reduced.
Japanese Laid-Open Patent Publication No. 2008-179135 discloses a recording method in which at an intersection where one drawing line among drawing lines of a character overlaps with another drawing line, the laser irradiation of the one drawing line at the intersection is inhibited in order to eliminate the intersection.
However, there is no disclosure in Japanese Laid-Open Patent Publication No. 2006-306063, Japanese Patent No. 3990891, and Japanese Laid-Open Patent Publication No. 2008-179135 of how to detect an overlapping portion or a turn-back portion in a character being drawn. The irradiation of a laser beam causes printing of strokes of a character with a certain amount of width on the medium, not printing of a point, and the strokes printed on the medium also have a limited thickness. For this reason, whether an overlap between strokes appears or not depends on the thickness of the strokes which are drawn by the laser irradiation.
FIGS. 2A-2C are diagrams illustrating examples of an overlap between strokes and a gap between strokes, which are influenced by the thickness of strokes and the size of characters.
FIG. 2A and FIG. 2B illustrate examples of a character in which the coordinates of points which constitute strokes are the same. In the example of FIG. 2A, the thickness of the strokes is small. In the example of FIG. 2B, the thickness of the strokes is large, and an overlap 230 between the strokes exists.
FIG. 2C illustrates examples of a character drawn by the method according to the related art in which the degree of overlap between strokes varies depending on the size of the character. In the original size of the character, by the method according to the related art, one of the line segments is divided into line segment parts to prevent overlapping of the line segments. However, the size of the character is not taken into consideration, and the manner of dividing is predetermined.
As illustrated in FIG. 2C, in the case of the enlarged character, the thickness of each stroke is unchanged from that in the case of the original size, the passing length of one line segment is enlarged, and a gap between the strokes appears.
On the contrary, in the case of the reduced character, the passing length of one line segment is reduced but the thickness of each stroke is unchanged, and an overlap between the strokes remains.
As described above, a line segment drawn by laser irradiation has a certain width, and therefore it is difficult for the methods and devices according to Japanese Laid-Open Patent Publication No. 2006-306063, Japanese Laid-Open Patent Publication No. 2004-341373 and Japanese Laid-Open Patent Publication No. 2008-179135 to certainly prevent occurrence of a gap or overlap between strokes at a crossing point, a turn-back point or the like of a character being drawn, which is influenced by the size of characters and the thickness of strokes.